We are endeavoring to understand factors that control gene expression in eukaryotic organisms. We are analyzing mutant murine plasmacytoma cells that have altered their level of immunoglobulin expression. Analyses are done at the DNA, RNA and protein levels. We are also preparing myeloma-spleen cell hybrids that are specific for the ozophenylarsonate hapten in order to understand at the organismic stage those factors that control Ig expression. These studies should aid in our understanding of differentiation in general and the immune response in particular.